This application relates to a method for controlling and reducing pesticide drift. More particularly, the application relates to a method for controlling and reducing pesticide drift caused by crystallization of pesticides from liquid spraying formulations.
Pesticides, which are chemicals poisonous to selected organisms, are used mainly in agriculture and forestry to control insects (insecticides), weeds (herbicides), fungi (fungicides) and other pests. Although the use of pesticides is viewed as beneficial, mainly because of increased agricultural yields, it is also responsible for many undesirable effects, including crop damage to nearby plants susceptible to the toxins, environmental pollution and human illnesses. Even when properly used, chemical pesticides have a number of unavoidable side effects. These problems due to inherent toxicity of pesticides are frequently amplified by the phenomenon of pesticide drift, i.e. the transport or movement of the toxic chemicals away from the intended area of deposition.
Pesticide drift is principally caused by airborne movement of particles or droplets produced when pesticides in the form of liquid formulations are sprayed over an area of application to ensure an even coverage of the area. Such an operation normally produces a wide range of droplet sizes including very small droplets which are buoyed up by air and may travel in the wind direction over substantial distances, causing an uncontrolled spread of pesticides.
In the United States approximately 300,000,000 acres receive pesticide application each year. Aerial applications, mainly by fixed wing aircrafts, are made to over 60% of this area (Ferguson, W. L., "Farmers Expenditures for Custom Pesticide Services", 1971 USDA AER #314, Nov. 1975). The extent of the problem caused by pesticide drift may be illustrated by damage claims reported by the insurance industry. For example, between June 1979 and July 1980, claims for crop damages caused by spray drift totalled U.S. $130,000,000, i.e. about 43 cents for every acre that was sprayed. Statistics of this kind do not even include environmental damage caused by the uncontrolled spread of pesticides.
As mentioned above, about 2/3 of the acreage receiving a pesticide application in the United States are aerially sprayed. When spraying formulations are applied to a field by an airplane, great quantities of small droplets are formed from normal atomization by spraying nozzles and by wind shear as the nozzles release the sprayed liquid through the air at 80 to 120 mph. These small droplets, typically less than 100 microns in diameter, may travel over large distances buoyed up by the air, thus causing a substantially uncontrollable spread of pesticides. Also as these small droplets fall toward the ground, the evaporation of the solvent or diluent, usually water, further reduces the size of the droplets. With evaporation the droplets shrink to particles composed mainly of the active ingredient and a small portion of the diluent and having frequently a size in the range of a few microns, i.e. forming an aerosol with practically unlimited drift ability. This small particle drift mechanism, due to evaporation of the diluent from the spray formulation, is common to all pesticides applied in the form of solutions and suspensions. Spray drift may also affect larger droplets. Since the extent of evaporation is mainly determined by the time during which droplets fall to the ground, droplets that are originally even substantially larger than 100 microns but that are caught up in the wing-tip vortices and flung 40 to 60 feet in the air have enough time to lose by evaporation most of the diluent while falling to the ground and to be reduced to small particles easily transportable over large distances by natural air movements. It is estimated that, due to drift, up to about 50% of the sprayed material never reaches the ground in the intended area of application when applied by an aircraft flying at a height of 7.5 feet above the ground and that this loss increases to about 80% if the application height is 30 feet (Avant, R. V. et al, "Pesticide Deposition vs Drift: The Alternatives", ASAE 79 -3556, 1979; Potts, F. S., Journ. Econ. Ent. 39: 716-712, 1946).
Pesticide drift may be also caused by the volatilization of the pesticide, as exemplified by esters of 2,4,-dichlorophenoxyacetic acid (2,4-D), in which case the pesticide itself may substantially vaporize from the applied material and drift in the gaseous form. This type of drift is frequently referred to as vapour drift.
Different factors, such as meteorological conditions, spray equipment, type of aircraft, chemical characteristics of pesticide, operator skill, etc. may influence all types of drift. Typically, the risk of drift is increased in aerial spraying and when high pressure, small nozzle tips are used in the spraying equipment. Also the spread of pesticides applied in the form of mists and dusts is usually more difficult to control.
When the pesticide is a solid material applied as a dust formulation, such as early DDT applications, there is a danger of the dust being moved even by a slight breeze over the target, either while being applied or after the application is made.
Some solid pesticides pose particular drift problems due to their strong tendency to crystallize from liquid formulations in which they are applied. When a liquid formulation comprising such a pesticide is sprayed and the diluent or solvent evaporates from the spray droplets, a large number of small crystals is rapidly formed. These crystals have their size and weight substantially reduced with respect to those of the original droplets and are easily moved by wind over large distances. Part of the material is deposited on the target site in droplets big enough to reach it before crystallization takes place. After evaporation of solvent also these droplets can release the active ingredient in the form of small crystals which may be moved by winds as if they were a dust formulation.
An example of such a pesticide is propanil (3',4'-di-chloropropionanilide) which is a contact herbicide of particular value for post-emergent control of grass weeds in rice, especially of barnyard grass. Barnyard grass is one of the most troublesome weeds of this crop. Propanil is a very effective and inexpensive herbicide for use with rice crops, but its strong tendency to crystallization and spray drift problems related to it, have resulted in very strict application controls.
Propanil is a water-insoluble substance supplied to the user in the form of an emulsifiable concentrate, typically as 50% by weight solution in a mixture of ketones. For spraying purposes this concentrate is diluted with water in a spray tank prior to spraying. It is believed that this dilution may cause partial crystallization of propanil in the spray tank, thus speeding up the process of crystallization after spraying, further accelerated by evaporation of water from the droplets.
To keep the water diluent from evaporating too quickly, various additives are typically incorporated into pesitcide spraying formulations. The literature indicates that petroleum oils have been used as spray additives for certain pesticides and crops but were found to be quite often phytotoxic. The use of other additives, such as surfactants, sticker/spreaders, water viscosity modifiers (polyvinyl alcohol, alginates, starches etc.) is also known. While these additives may produce measurable differences in evaporation rates and viscosities of spraying compositions, they do not significantly suppress or reduce pesticide drift.
Vegetable oils, such as cottonseed oil are sometimes used as spraying additives. According to Akessen, Yates, Brazelton and Natwick (paper No. 83-1510, "Laboratory and Field Studies on Vegetable Oil-Based Spray Applications", American Society of Agricultural Engineers, Chicago, Dec. 13-16, 1983) when cottonseed oil was used in combination with propanil concentrate, as the ketone evaporated, crystals of propanil appeared as "striations" on the oil surface. The formation of crystals of propanil was also observed from a formulation comprising 20% of cottonseed oil, 10% of herbicide and 70% of water (by volume). As can be seen from the above, vegetable oils are not efficient in preventing the crystallization of propanil and, consequently, spray drift.